Windows Presentation Foundation (WPF)

Windows Presentation Foundation (WPF) provides developers with a unified programming model for building rich Windows smart client user experiences that incorporate UI, media, and documents.
Source: MSDN.Microsoft

What's good about WPF?
Programming in WPF makes it easier for me to build greater UI especially with the use of another developer tool which is Microsoft Expression Blend. 
There is a behind the code - .cs where you can code the functionality of your program/project and there is .xaml where you can code your UI.

What is XAML?

XAML is a declarative markup language. As applied to the .NET Framework programming model, XAML simplifies creating a UI for a .NET Framework application. You can create visible UI elements in the declarative XAML markup, and then separate the UI definition from the run-time logic by using code-behind files, joined to the markup through partial class definitions. XAML directly represents the instantiation of objects in a specific set of backing types defined in assemblies. This is unlike most other markup languages, which are typically an interpreted language without such a direct tie to a backing type system. XAML enables a workflow where separate parties can work on the UI and the logic of an application, using potentially different tools.

When represented as text, XAML files are XML files that generally have the .xaml extension. The files can be encoded by any XML encoding, but encoding as UTF-8 is typical.

Source: msdn.microsoft